There are many applications in the material processing industry where it is necessary to monitor and control the composition of a multi-phase fluid medium to obtain certain desired end product qualities. Typical examples are the emulsification and crystallization processes, both of which involve liquid fluid suspensions. Other examples include fluidized beds and dry grinding circuits which involve air suspensions.
In an emulsification process two immiscible liquids are forcefully mixed to produce a distribution of individual droplets of the one liquid suspended within the other. The size distribution of these droplets directly determines the stability of the emulsion. By measuring this distribution one can control the mixing process and the resulting stability of the emulsion.
A crystallization process is used to separate one material dissolved within another. For example, in the batch crystallization of sugar, impure sugar solids are dissolved in a heated, aqueous solution. When this mixture reaches a supersaturated state, the temperature is lowered, which causes the individual sugar molecules to crystallize out of solution by depositing layer-by-layer in a defined pattern along the crystal lattices of the crystalline seed particles. The seed particles are either formed by spontaneous nucleation or are introduced into the crystallizer at the beginning of the process.
The material inside the crystallizer is in multi-phase form. In the case of sugar crystallization, inside a crystallizer apparatus, the solution is typically in a vapor phase (water vapor intermixed with air), a liquid phase (water and the dissolved sugar/molasses mixture), and a solid phase (the sugar crystals). The efficiency and efficacy of the crystallization process critically depends on the extent of the supersaturation, the size and numbers of the seed particles and the dynamics of the mixing action, i.e. the balance between the growth rate and attrition (grinding) effects.
There are typically three physical zones inside a crystallizer, (i) the stable (unsaturated) zone where crystallization is not possible, (ii) the metastable (supersaturated) zone where spontaneous crystallization is improbable, and (iii) the unstable or labile (supersaturated) zone, where spontaneous crystallization is probable. Inside zone (ii) and zone (iii), the material molecules can begin to exhibit molecular alignment within certain clusters (or `swarms`) prior to the occurrence of nucleation and the formation of crystalline solids.
As described in Kerker, The Scattering of Light, Academic Press, New York, 1969, density and concentration fluctuations in the solution cause index of refraction changes in the solution, which in turn causes scattering of light. While it is generally very difficult to observe these fluctuations, they do become detectable by measuring properties such as double refraction and the production of interference patterns of polarized light caused by light scattering at the boundaries of these swarms. The detection and measurement of these swarms are an important indicator of the state of supersaturation of the liquid and can predict the onset of spontaneous nucleation.
U.S. Pat. No. 4,871,251 to Preikschat et al. entitled Apparatus and Method for Particle Analysis, commonly assigned to the assignee of the present invention and incorporated by reference, describes an in-line particle analysis system with a measuring window placed into a flow stream. While this type of apparatus does show relative changes in the particle size distribution, it is not able to provide accurate quantitative data on particle size and shape, or the liquid `swarms` that may be present prior to nucleation. The apparatus described therein evolved into another apparatus the subject of U.S. Pat. No. 5,124,265.
The material characteristics of a multi-phase solution (also referred to as a slurry) can vary widely inside a reactor vessel. For example, inside a continuously operated sugar crystallizer, large heat exchanger plates are mounted to heat (or cool) the super-saturated sugar solution. In addition, large agitator plates move the smaller seed crystals into the `meta-stable` crystallization growth zone and the larger adult crystals into a separation zone by sedimentation. The final crystal size distribution as measured by a probe depends on a number of factors such as the distance between the probe and the agitator blades, the position of the probe window relative to the flow and the overall probe location. These dynamic variables can all influence the probe readings and must be taken into consideration to properly interpret the measured results.
The above described applications involve liquid suspensions. In many industrial processes air suspensions are frequently used to move, dry or agglomerate a flow of particles. Typical examples are pneumatic conveying systems, fluidized bed suspensions and spray dryers. The former two are used to convey a flow of dry particles. The latter is used to dry and agglomerate fine particles. In all of these cases it may again be desirable to measure or control the size of the particle stream. In the case of pneumatic conveying systems the particle flow velocity may be as high as 100 feet per second. This requires that optical measuring systems have high response speeds in order to monitor the size and shape of individual particles.
In the case of liquid suspensions, the hydrodynamic flow conditions can vary greatly, depending on material characteristics (viscosity, size, and distribution) and the boundary conditions, e.g. distance from the walls of a process vessel. At the walls of a process vessel, there are "dead zone layers" which mask the "real" process conditions inside the vessel. There is also typically a thin turbulent boundary layer at the walls of pipelines used for liquid slurry transport. In the case of pulp flow at 2-5% consistency, this turbulent layer can be as thick as 1 mm. The turbulent "rolling" action of this layer prevents the larger particles from hitting against the walls of the pipeline. The velocity profile, taken radially across a pipe line, is highly peaked with maximum values near the center of the pipe and small values at the walls. The profile is also influenced by the material parameters, the flow speed and the Reynold's number. If the viewing window is placed in line with and parallel to the outer walls, the particle size distribution measured at the walls will differ considerably from that directly inside the flow. The difference becomes larger with increasing slurry concentrations, and becomes significant at concentrations of 20-30%, as typically encountered inside industrial processes.
In the apparatus described in U.S. Pat. No. 4,871,251, particle size distribution is measured by a series of linear, one-dimensional scans of random particle chord lengths. This is convened into a number (or frequency) distribution of these (random chord) lengths, and provides a good relative indication of particle size in the lower size ranges. The measurement does not provide shape information or absolute size measurement in the larger crystal size ranges.
The measurement of crystal shapes can be important, because it is a predictor of material properties. Diamond and coal, for example, are both made of carbon but have an entirely different crystal structure and material properties. By monitoring the shape of newly grown crystals, control of process conditions can be facilitated, resulting in higher product quality.
Microscope imaging techniques are presently the only known method to measure both particle shape and absolute particle size. There are a number of on-line vision analyzer systems, for example:
1. the continuous, on-line analyzer marketed by Flow Vision, Inc under the trademark PED-1 and PED-11, used as an inspection system for plastic melts; and PA0 2. the system marketed by J. M. Canty Associates, Inc. described in U.S. Pat. No. 4,965,601, used as a wall-mounted inspection system. PA0 a. the viewing window must be capable of being placed directly into a process flow to measure the dynamic changes taking place inside the process; PA0 b. the system must provide fast signal counting and processing rates to measure process information in real time; PA0 c. it must withstand operation at high pressures and over a wide range of process temperatures from cryogenic (-50.degree. C.) to high temperatures (+150.degree. C.) ; PA0 d. include the ability to withstand highly toxic and corrosive materials, survive at high vibration levels and wide range of environmental conditions, and be designed for operation in hazardous environments; PA0 e. include means to extract the probe from the process, while the process is in operation, to remove and clean the probe window; PA0 f. must be able to operate at very high slurry concentrations, as typically encountered at full in-line process concentrations; and PA0 g. it must be able to measure particles over a wide size range from 1 .mu.m to 3000 .mu.m (3 mm).
These systems are used with the viewing window mounted flush with the walls of the reaction vessel (or pipe line) and mainly to measure impurity levels in relatively dilute fluids, like a plastic melt in the plastics extrusion industry. Under these conditions, imaging systems can readily discriminate between individual particles and a high contrast background (either dark in the case of back-scattering, or light in the case of a transmission geometry). They do not readily lend themselves to be used in a probe configuration.
The amount of light back-scattered from a fluent medium depends on the particle concentration, the particle size distribution, the index of refraction of the various phase constituents in the multi-phase medium, the shape of the particles, the turbidity and the absorption coefficient of the medium. At high process concentrations, the material densities are so large that there is little space between the adjacent particles. Because it requires a million small particles (of 1 .mu.m in diameter) to make up the same weight as a single large (100 .mu.m) particle, slurries at high process concentrations contain many more fine particles by number than large particles. When an unfocused, white light source is used as a source of illumination, the light will be multiple scattered from the many individual small particles and will become highly diffused. This makes it difficult to resolve individual particles and has always limited the utility of imaging systems for in-line process applications at high slurry concentrations.
If the material concentration within the fluid medium is sufficiently high, the light beam will be multiple scattered by more than one particle in the beam. This transition from single to multiple scattering is gradual and occurs over a region where about 26% of the light in a collimated beam is obscured by particles in the light beam (see Jones, A. R., Scattering of Electromagnetic Radiation in Particulate Laden Fluids, Prog. Energy Combust. Sci., 5, 73-96). In many industrial applications the material concentrations can be so high that over 50% of the light gets obscured in a distance of only a few microns into a slurry flow. Multiple scattered light becomes randomized and depolarized. Further, using a white light source will wash out any features in the scattered light and make it very difficult to see any shadows or contrast between neighboring particles.
Sensor probes intended for in-line use in the chemical processing industry have to meet a number of other requirements, most of which have not been previously addressed until the present invention,
The above criteria are necessary conditions for a measuring system to be useful in the chemical processing industry, and it is the objective of this invention to provide this capability.